1. Field of the Invention
The present invention relates to the structure of an ink jet head, and relates particularly to a technology for controlling the pressure in the pressure generating chamber that applies an ejecting pressure to the ink contained in the chamber.
2. Description of the Related Art
In general, an ink jet head comprises a pressure generating chamber for applying pressure to ink to eject the ink from a nozzle. One end of the pressure generating chamber is typically connected to an ink tank through an ink supply path, and the other end to a nozzle opening from which the ink drops are ejected. Part of the pressure generating chamber is made to be easily deformed and functions as a diaphragm. This diaphragm is elastically displaced by an electromechanical conversion means to generate the pressure that ejects ink drops from the nozzle opening.
Recording apparatuses using this type of ink jet head offer outstanding operating characteristics, including low operating noise and low power consumption. They are widely used as hard copy output devices for a variety of information processing devices. As the performance and functionality of information processing devices has improved, demand has also risen for even higher quality and speed printing both text and graphics. This has made urgent the development of technologies enabling even finer and smaller ink drops to be ejected consistently at even higher frequencies i.e., higher printing speed.
(1) Ink eject frequency
Because of the structure of the ink jet head as described above, after ink ejection, vibration remains in the ink inside the pressure generating chamber (also called the ink chamber because it is filled with ink; hereafter "ink chamber"). This residual vibration can easily result in the formation of undesirable ejected ink droplets (also called "satellites"). To avoid this, the flow resistance of the ink supply path connecting the ink chamber and ink tank is conventionally set high as a means of accelerating attenuation of residual ink vibration. However, if the flow resistance of the ink supply path is high, the refill supply rate of ink to the ink chamber, after ink ejection, drops, thereby lowering the maximum ink eject frequency, and thus lowering the printing speed of the printing device.
The applicants thus developed and disclosed in JP-A-H6-320725 (1994-320725) a technology for forming a thin-wall part in the diaphragm to create a flexible wall that deforms according to the pressure inside the ink chamber. This thin-wall part is used to absorb residual ink vibration in the ink chamber as a means of avoiding undesirable ink ejection or satellite emissions. It is therefore not necessary to set the flow resistance of the ink supply path high because ink ejection does not occur even if there is residual ink vibration, and the ink ejecting frequency can therefore be increased.
With regard to the technology described in JP-A-H6-320725 (1994-320725), the compliance (i.e., volume change per unit pressure) of the ink chamber increases due to the thin-wall part of the diaphragm. While this reduces satellites, the ejecting speed required for stable ink ejection cannot be obtained because the pressure generated by the diaphragm for ink ejecting is not used effectively for propelling the ink drops. Furthermore, when the diaphragm drive force is increased to assure sufficient ejecting speed, a higher drive voltage is required. This, in turn, increases both the size of the drive device and power consumption.
(2) Improving image quality with technologies for varying droplet size
Expressing various density gradations by changing the size of the ink droplets formed on the recording medium is a preferred means of improving image quality. The size of the ink droplets output by any recording apparatus (printer) using an ink jet head is determined by various factors, one of which is the size (also called "ink ejection mass") of the ink drops ejected by the ink jet head.
A technology providing plural electrostrictive means of different sizes in the ink chamber, and separately controlling and driving these electrostrictive means to eject ink droplets of various sizes, is described in JP-A-S55-79171 (1980-79171).
When the technology described in JP-A-S55-79171 (1980-79171) is applied, each of the plural, different size actuators used to deform the diaphragm must be independently driven, resulting in increasing the number of wires needed, and thus making it difficult to achieve a high nozzle density. The number of drivers also increases because of the need to separately drive each actuator, and this makes it difficult to reduce the device size.
(3) Improving image quality through a high droplet density
Most ink jet heads usually have plural nozzles arrayed in a straight line. Printing devices using such ink jet heads output two-dimensional images by moving the ink jet head across the recording medium in a direction roughly perpendicular to this nozzle line. Therefore, to achieve high image quality by increasing the ink droplet density, it is necessary to reduce the distance between adjacent nozzles (also known as the "nozzle pitch").
An ink jet head using an electrostatic actuator developed and manufactured by the applicants can be manufactured using a production process similar to that used for semiconductor manufacture, and is one of the technologies best suited to achieving a high ink droplet density. The basic structure of this ink jet head is described in JP-A-H5-50601 (1993-50601), and can be used to reduce the nozzle pitch without changing the size of the ink drops by narrowing the width and increasing the length of the ink chamber.
An ink jet head using electrostatic actuators as described in JP-A-H5-50601 (1993-50601) can decrease the nozzle pitch without changing the size of the ink droplets. In this case, however, the diaphragm compliance increases significantly as described below, and an extremely high voltage is therefore required to drive the electrostatic actuator. In general, the load on the drive device increases as the drive voltage increases, and measures to prevent unnecessary radiation are difficult. As a result, it is difficult to actually use this type of ink jet head in a printing device.